Ever looked at your bedside table and thought–what can I rest on there? What can I read before bed that won’t give me nightmares, but may potentially arouse my intellect, twang on my heartstrings, or initiate some kind of pillow talk with my loved one that isn’t about who dropped a kleenex in the laundry machine? Perhaps you’re a science professor, or a lover of all things quirky…

Super-soft fur and slippery skin. Or is that lickable nipples and arguable kin? Or fun-filled frolicking in the name of sin? Whatever we call it, however high it flies on the rarefied notes of an aesthetic sensibility or low it sinks in the aftermath of familial responsibilities and limited options, the urge to merge—the lustful morass of feelings, emotions, and relationships around which mammalian sexuality swirls—begins and ends with bodies. To understand it, we must do a little time traveling. Fortunately, time travel itself is, so far, impossible. Fortunately because, if you were to go back and fall in lust with a fur-clad cave hunk or hottie, you might sire or give birth to a boy who grows to a man who kills your own ancestors. That would not only be a science-fiction paradox but also deprive you of the pleasure of reading this book.

But if we can’t go turn the clock back, or depend on evolutionists’ just-so stories, how can we find out what our ancestors were up to?

A powerful tool in reconstructing probable ancestral sex lives—less “just-so” than “might-be-so” stories—is comparative anatomy. By looking at now-living related organisms, we can see what traits they share and backtrack to determine probable features of an ancestor. The same can be done by comparing behavior, mating systems, and DNA sequences. There will be false leads, but, like the weight of circumstantial evidence carefully employed to re-create a crime scene, we can come up with a plausible picture. And unlike the prosecuting attorney, a scientist does not have to prove his case beyond a reasonable doubt. The continuum stretches not between crime and punishment, but between curiosity and discovery. New evidence will not get anyone out of prison, but it may release us from the subtler incarceration of received opinion.

In the 1980s, and although in the center of a full house near the front row, I walked out of a lecture by a creationist who was trying to make fun of evolutionists during the course of his slide show. “Evolutionists want you to believe,” he said, flashing a crude cartoon of a cow by the seashore, “that this”— and then our intrepid advocate flashed forward to a picture of a great whale in the water—“turned into this.”

Ovid, in his Metamorphoses, recounts some startling transformations. But a cow turning into a whale is not one of them, any more than it is for evolutionists. Caricatures and straw men do not an argument make. It is true that the ancestors of whales, dolphins, walruses, and seals were likely land mammals—more like goats than cows but in truth neither. Embryonic humans resemble embryonic mice and chickens— all three in utero look literally fishy: We have gill slits and tails before we come out of our mothers. Why would a creator give us gill slits in the womb, unless he used evolution to create, or was a prankster?

Anatomical similarities often reveal shared evolutionary roots. The evidence of common lineage is not limited to embryos. It is literally in our bones. The foreleg of a horse, the wing of a bat, the flipper of a whale, and the arm of a Moulin Rouge dancer all share a similar skeletal infrastructure.

Even the more honest creationist tactic of finding God in the gaps in the fossil record—emphasizing missing links— misses the point: What is remarkable is not what separates, but what connects us. Like a giant jigsaw all scientists are working on concurrently, missing pieces continue to be found. And they are profound. The 1850 Berlin discovery of the winged reptile Archaeopteryx would have delighted Darwin, in whose time the jigsaw puzzle, mostly due to the dearth of paleontological piece finders, had just begin. Today a slew of new fossils of feathered dinosaurs have been unearthed in China. Indeed, paleontologists now classify birds as dinosaurs: They lay eggs, have scales on their feet, and are technically reptiles. Paleontologist Jack Horner (an inspiration for the book/film Jurassic Park) even claims to be able to produce a modern-day mini dinosaur by interfering with embryonic development of a chicken, a small featherless dinosaur with teeth.

We are backboned animals with anatomical and sexual characteristics similar to other organisms that share our ancestry. The coccyx, the little tailbone at the bottom of our spine, serves no purpose for us now but it did when our simian ancestors swung from the trees. A grasping tail is an excellent tool if you are used to clinging to a branch as you call out for a furry friend. The great and lesser apes and Old World (African and Asian) monkeys all lack grasping tails. Some of the smaller New World (North and South American) monkeys, the smallest of which is the pygmy marmoset, a paltry lightweight at five ounces, have grasping tails. Unlike bigger Old World monkeys, the New World simians rarely come down to the ground, except for the occasional nut or cricket, preferring to scamper about from branch to branch (some, such as the marmosets, feeding directly on tree sap with special bark-piercing teeth) in the tropical forests in southern Mexico, Central and South America. Although it’s impossible for landlubbers to keep full account of the sixty-odd species of New World monkeys, their sexual and social relationships vary, with, for example, male tamarins and marmosets (whose females typically give birth to twins) carrying the infants most of the time, whereas daddy capuchins (the famous organ grinder monkeys) do not tend to take care of their offspring; some New World monkey species have harems with one male and several female consorts, while others, such as the callicebus monkeys (titis), tend to form long-term monogamous relationships. A similar variety marks the apes and Old World monkeys, who are more closely related to us.

The Platyrrhini, the ancestral stock that became the New World monkeys, may have arrived in South America on floating chunks of vegetation. They could have traveled on a natural raft like the floating mangrove forest islands that violent storms sometimes break off the coast of Africa. Geographic isolation—the separation of populations as the result of such events—was probably a major factor in the evolution of primates. A floating island, earthquake-separated patch of jungle, or primate tribe following fruit trees into a remote and distant valley and remaining there may separate members of a genetic stock. Physically separated, they no longer interbreed. Ultimately troops and tribes went their own way, evolving to the point that they could not form fertile offspring with members of the ancestral lineage even if they were still able and willing to mate with them. In this way new species, including our ancestors—who were mating long before there were humans—formed.

Genetic and fossil evidence suggests that the flat-nosed, branch-swinging New World monkeys split from the Old World monkeys—baboons, macaques, and many more— some forty million years ago. The island-hopping ancestors to the New World monkeys would have been aided in their journey on floating clumps of vegetation to the New World because Africa and South America were closer together thirtyseven million years ago in the Oligocene epoch.

The Old World monkeys, like us and apes, are catarrhines (Greek for “hook-nosed”) with downward-pointing nostrils. The biggest superficial difference among the three great primate groups closest to us—the Old World monkeys, the New World monkeys, and the African and Asian apes—is in the tails. The catarrhines, when they have tails, can’t hang, clutch, or hug with them as can the broad-nosed platyrrhines. Old World monkeys and the apes, like us, despite some vestiges here and there, have outgrown them. This could be because, unused, any changes that shortened tails had no material effect on survival, as our Old World ancestors gave up navigating the arboreal jungle gym for splendoring in the grass. Use it or lose it. But the true tale of the tail, as usual, is probably more complex. The coccyx, uterine tail, and occasional birth of children with tails indubitably suggest that our ancestors had tails and that, if we are made in God’s image and the devil an angel, they may also have been so endowed.

By looking more closely at the members of our evolutionary group, we can glean something of our shared ancestors’ sex lives—the erotic ape matrix of which human sex lives, despite their variety, are only a perhaps passing variation.

The evolutionary family Hominidae to which humans belong includes two species of chimp, the common and bonobo; three subspecies of gorilla, western and eastern lowland and mountain gorillas; and two species of orang, the Bornean and Sumatran. Immunological studies in the 1960s showed that the African apes are far more closely related to us than to Old World monkeys.

Although not directly answering the famous barb of Bishop Samuel Wilberforce in his 1860 debate with evolutionist Thomas Henry Huxley as to whether it was through his grandmother or his grandfather that he claimed descent from a monkey, a combination of fossil, immunological, and genetic evidence suggests the Old World monkeys split from the great ape lineage of which we are part some thirty million years ago. Various methodologies suggest that the orangutan line split off from the other great apes about fifteen million years ago, the gorillas about seven million years ago, and humans from common ancestors with chimps some five million years ago.

A recent genetic study that offers a clue about the fur gap between people and the other hominoids has to do with a protein. As shampoo ads sometimes mention, proteins are a major constituent of hair. In fact our bodies are mostly protein—blood, skin, organs, toenails, hair, and so on are all made of proteins. The main sort of proteins in hair are called keratins. The journal Human Genetics suggests that one of these proteins, human type I hair keratin, appears to be coded for by a gene that may have been inactivated some time after the divergence of Pan (chimps) and Homo (modern and extinct humans). This gene is one of the eighty that have been lost— thirty-six of which code for olfactory receptors allowing a better sense of smell. A disproportionate number of the other genes lost had to do with immune response, perhaps reflecting different pathogens in the primeval environments in which we and our soul-sister lineage evolved. But losing the type I hair keratin gene may have been the immediate cause of human body hair loss. The massive thinning and loss of our ancestors’ body hair is estimated to have occurred about 250,000 years ago, very recently in geological terms.

But behind the immediate genetic cause may well lie a deeper cause. Evolutionary biologists distinguish between ultimate and proximate cause. Proximate cause refers to immediate chemical or physical cause. Ultimate cause refers to evolutionary factors that can no longer be directly observed. One of the first to postulate an ultimate cause for human hair loss was the author Desmond Morris, who intriguingly suggested that sex was part of the story of why our ancestors lost their fur.